Chapter Four : Processor Management

1
Chapter Four Processor Management

• Introduction
• Job Scheduling Versus Process Scheduling
• Process Scheduler
• Process Identification
• Process Status
• Process State
• Accounting
• Process Scheduling Policies
• Process Scheduling Algorithms
• Cache Memory
• Interrupts

• Job Scheduling
• Process Scheduling
• Interrupt Management

2
How Does Processor Manager Allocate CPU(s) to
Jobs?

• Process Manager performs job scheduling, process
  scheduling and interrupt management.
• In single-user systems, processor is busy only
  when user is executing a jobat all other times
  it is idle.
• Processor management is simple.
• In multiprogramming environment, processor must
  be allocated to each job in a fair and efficient
  manner.
• Requires scheduling policy and a scheduling
  algorithm.

3
Some Important Terms

• Program inactive unit, such as a file stored on
  a disk.
• To an operating system, a program or job is a
  unit of work that has been submitted by user.
• Job is usually associated with batch systems.
• Process (task) active entity, which requires a
  set of resources, including a processor and
  special registers to perform its function.
• A single instance of an executable program.

4
Important Terms - 2

• Thread of control (thread) a portion of a
  process that can run independently.
• Processor ( CPU, central processing unit) part
  of machine that performs calculations and
  executes programs.
• Multiprogramming requires that the processor be
  allocated to each job or to each process for a
  period of time and deallocated at an
  appropriate moment.

5
Job Scheduling vs. Process Scheduling

• Processor Manager has 2 sub-managers
• Job Scheduler
• in charge of job scheduling.
• Process Scheduler
• in charge of process scheduling.

6
Job Scheduler

• High-level scheduler.
• Selects jobs from a queue of incoming jobs.
• Places them in process queue (batch or
  interactive), based on each jobs
  characteristics.
• Goal is to put jobs in a sequence that uses all
  systems resources as fully as possible.
• Strives for balanced mix of jobs with large I/O
  interaction and jobs with lots of computation.
• Tries to keep most system components busy most of
  time.

7
Process Scheduler

• Low-level scheduler assigns the CPU to execute
  processes of those jobs placed on ready queue by
  Job Scheduler.
• After a job has been placed on the READY queue by
  Job Scheduler, Process Scheduler that takes over.
• Determines which jobs will get CPU, when, and for
  how long.
• Decides when processing should be interrupted.
• Determines queues job should be moved to during
  execution.
• Recognizes when a job has concluded and should be
  terminated.

8
CPU Cycles and I/O Cycles

• To schedule CPU, Process Scheduler uses common
  trait among most computer programs they
  alternate between CPU cycles and I/O cycles.

9
Poisson Distribution Curve

• I/O-bound jobs (such as printing a series of
  documents) have many brief CPU cycles and long
  I/O cycles.
• CPU-bound jobs (such as finding the first 300
  prime numbers) have long CPU cycles and shorter
  I/O cycles.
• Total effect of all CPU cycles, from both
  I/O-bound and CPU-bound jobs, approximates a
  Poisson distribution curve.
• Figure 4.1

10
Processor Manager Middle-Level Scheduler

• In a highly interactive environment theres a
  third layer
• called middle-level scheduler.
• Removes active jobs from memory to reduce degree
  of multiprogramming and allows jobs to be
  completed faster.

11
Job and Process Status

• Job status -- one of the 5 states that a job
  takes as it moves through the system.
• HOLD
• READY
• WAITING
• RUNNING
• FINISHED

12
Job and Process Status
Admitted
Finished
Interrupt
Exit
Scheduler dispatch
I/O or event completion
I/O or event wait
Handled by Process Scheduler Handled by Job
Scheduler
13
Transition Among Process States

• HOLD to READY Job Scheduler using a predefined
  policy.
• READY to RUNNING Process Scheduler using some
  predefined algorithm
• RUNNING back to READY Process Scheduler
  according to some predefined time limit or other
  criterion.
• RUNNING to WAITING Process Scheduler and is
  initiated by an instruction in the job.
• WAITING to READY Process Scheduler and is
  initiated by signal from I/O device manager that
  I/O request has been satisfied and job can
  continue.
• RUNNING to FINISHED Process Scheduler or Job
  Scheduler.

14
Process Control Block (PCB)

• Process Control Block (PCB) -- data structure
  that contains basic info about the job
• Process identification
• Process status (HOLD, READY, RUNNING, WAITING)
• Process state (process status word, register
  contents, main memory info, resources, process
  priority)
• Accounting (CPU time, total amount of time, I/O
  operations, number input records read, etc.)

15
PCBs and Queuing

• PCB of job created when Job Scheduler accepts it
• updated as job goes from beginning to
  termination.
• Queues use PCBs to track jobs.
• PCBs, not jobs, are linked to form queues.
• E.g., PCBs for every ready job are linked on
  READY queue all PCBs for jobs just entering
  system are linked on HOLD queue.
• Queues must be managed by process scheduling
  policies and algorithms.

16
Process Scheduling Policies

• Before operating system can schedule all jobs in
  a multiprogramming environment, it must resolve
  three limitations of system
• finite number of resources (such as disk drives,
  printers, and tape drives)
• some resources cant be shared once theyre
  allocated (such as printers)
• some resources require operator intervention
  (such as tape drives).

17
A Good Scheduling Policy

• Maximize throughput by running as many jobs as
  possible in a given amount of time.
• Maximize CPU efficiency by keeping CPU busy 100
  of time.
• Ensure fairness for all jobs by giving everyone
  an equal amount of CPU and I/O time.

• Minimize response time by quickly turning around
  interactive requests.
• Minimize turnaround time by moving entire jobs
  in/out of system quickly.
• Minimize waiting time by moving jobs out of READY
  queue as quickly as possible.

18
Interrupts

• There are instances when a job claims CPU for a
  very long time before issuing an I/O request.
• builds up READY queue empties I/O queues.
• Creates an unacceptable imbalance in the system.
• Process Scheduler uses a timing mechanism to
  periodically interrupts running processes when a
  predetermined slice of time has expired.
• suspends all activity on the currently running
  job and reschedules it into the READY queue.

19
Preemptive Non-preemptive Scheduling Policies

• Preemptive scheduling policy interrupts
  processing of a job and transfers the CPU to
  another job.
• Non-preemptive scheduling policy functions
  without external interrupts.
• Once a job captures processor and begins
  execution, it remains in RUNNING state
  uninterrupted.
• Until it issues an I/O request (natural wait) or
  until it is finished (exception for infinite
  loops).

20
Process Scheduling Algorithms

• First Come First Served (FCFS)
• Shortest Job Next (SJN)
• Priority Scheduling
• Shortest Remaining Time (SRT)
• Round Robin
• Multiple Level Queues

21
First Come First Served (FCFS)

• Non-preemptive.
• Handles jobs according to their arrival time --
  the earlier they arrive, the sooner theyre
  served.
• Simple algorithm to implement -- uses a FIFO
  queue.
• Good for batch systems not so good for
  interactive ones.
• Turnaround time is unpredictable.

22
FCFS Example

• Process CPU Burst (Turnaround Time)
• A 15 milliseconds
• B 2 milliseconds
• C 1 millisecond
• If they arrive in order of A, B, and C.
• What does the time line look like?
• Whats the average turnaround time?

23
Shortest Job Next (SJN)

• Non-preemptive.
• Handles jobs based on length of their CPU cycle
  time.
• Use lengths to schedule process with shortest
  time.
• Optimal gives minimum average waiting time for
  a given set of processes.
• optimal only when all of jobs are available at
  same time and the CPU estimates are available and
  accurate.
• Doesnt work in interactive systems because users
  dont estimate in advance CPU time required to
  run their jobs.

24
Priority Scheduling

• Non-preemptive.
• Gives preferential treatment to important jobs.
• Programs with highest priority are processed
  first.
• Arent interrupted until CPU cycles are completed
  or a natural wait occurs.
• If 2 jobs with equal priority are in READY
  queue, processor is allocated to one that arrived
  first (first come first served within priority).
• Many different methods of assigning priorities by
  system administrator or by Processor Manager.

25
Shortest Remaining Time (SRT)

• Preemptive version of the SJN algorithm.
• Processor allocated to job closest to completion.
• This job can be preempted if a newer job in READY
  queue has a time to completion that's shorter.
• Cant be implemented in interactive system --
  requires advance knowledge of CPU time required
  to finish each job.
• SRT involves more overhead than SJN.
• OS monitors CPU time for all jobs in READY queue
  and performs context switching.

26
Context Switching Is Required by All Preemptive
Algorithms

• When Job A is preempted
• All of its processing information must be saved
  in its PCB for later (when Job As execution is
  continued).
• Contents of Job Bs PCB are loaded into
  appropriate registers so it can start running
  again (context switch).
• Later when Job A is once again assigned to
  processor another context switch is performed.
• Info from preempted job is stored in its PCB.
• Contents of Job As PCB are loaded into
  appropriate registers.

27
Round Robin

• Preemptive.
• Used extensively in interactive systems because
  its easy to implement.
• Isnt based on job characteristics but on a
  predetermined slice of time thats given to each
  job.
• Ensures CPU is equally shared among all active
  processes and isnt monopolized by any one job.
• Time slice is called a time quantum
• size crucial to system performance (100 ms to 1-2
  secs)

28
If Jobs CPU Cycle lt Time Quantum

• If jobs last CPU cycle job is finished, then
  all resources allocated to it are released
  completed job is returned to user.
• If CPU cycle was interrupted by I/O request, then
  info about the job is saved in its PCB it is
  linked at end of the appropriate I/O queue.
• Later, when I/O request has been satisfied, it is
  returned to end of READY queue to await
  allocation of CPU.

29
Time Slices Should Be ...

• Long enough to allow 80 of CPU cycles to run to
  completion.
• At least 100 times longer than time required to
  perform one context switch.
• Flexible -- depends on the system.

30
Multiple Level Queues

• Not a separate scheduling algorithm.
• Works in conjunction with several other schemes
  where jobs can be grouped according to a common
  characteristic.
• Examples
• Priority-based system with different queues for
  each priority level.
• Put all CPU-bound jobs in 1 queue and all
  I/O-bound jobs in another. Alternately select
  jobs from each queue to keep system balanced.
• Put batch jobs background queue interactive
  jobs in a foreground queue treat foreground
  queue more favorably than background queue.

31
Policies To Service Multi-level Queues

• No movement between queues.
• Move jobs from queue to queue.
• Move jobs from queue to queue and increasing time
  quantums for lower queues.
• Give special treatment to jobs that have been in
  system for a long time (aging).

32
Cache Memory

• Cache memory -- quickly accessible memory thats
  designed to alleviate speed differences between a
  very fast CPU and slower main memory.
• Stores copy of frequently used data in an easily
  accessible memory area instead of main memory.

33
Types of Interrupts

• Page interrupts to accommodate job requests.
• Time quantum expiration.
• I/O interrupts when issue READ or WRITE command.
• Internal interrupts (synchronous interrupts)
  result from arithmetic operation or job
  instruction currently being processed.
• Illegal arithmetic operations (e.g., divide by
  0).
• Illegal job instructions (e.g., attempts to
  access protected storage locations).

34
Interrupt Handler

• Describe store type of interrupt (passed to
  user as error message).
• Save state of interrupted process (value of
  program counter, mode specification, and
  contents of all registers).
• Process the interrupt
• Send error message state of interrupted process
  to user.
• Halt program execution
• Release any resources allocated to job are
  released
• Job exits the system.
• Processor resumes normal operation.

35
Key Terms

• aging
• cache memory
• context switching
• CPU-bound
• first come first served
• high-level scheduler
• I/O-bound
• indefinite postponement
• internal interrupts

• interrupt
• interrupt handler
• Job Scheduler
• job status
• low-level scheduler
• middle-level scheduler
• multiple level queues
• multiprogramming
• non-preemptive scheduling policy

36
Key Terms - 2

• preemptive scheduling policy
• priority scheduling
• process
• Process Control Block
• Process Scheduler
• process scheduling algorithm
• process scheduling policy process status

• processor
• queue
• response time
• round robin
• shortest job next (SJN)
• shortest remaining time
• synchronous interrupts
• time quantum
• time slice
• turnaround time